Chemistry Reference
In-Depth Information
Summary.
Many models have been proposed for the mechanisms involved in
PS formation, but they generally have dealt only with limited aspects of this very
complex system. No single theory is yet close to a globally quantitative description of
the extremely rich and complex nature of the reactions and resulting morphological
evolution of the silicon electrode surface involved in the formation of PS. Generally,
some models can logically correlate certain theories in solid state and electrochemistry
with some aspects of the phenomena but not with other aspects. Also, the morphology
of PS has such a rich nature that every time a model is proposed to describe a set of
features, further details that could not be predicted by the model are discovered. Given
what has occurred in the last several decades during which there has been continuous
discovery of new morphological features, it can be expected that new unexpected mor-
phological features of PS may be discovered in the future. The situation about the for-
mation mechanisms of PS is somewhat like the age-old tale about a group of blind
people trying to describe an elephant, each provides a partial truth of the whole reality.
We are still not at the stage where we can quantitatively model the mechanisms of PS
formation and PS morphology with reasonable accuracy.
8.6.2. Analysis of the Mechanistic Aspects Involved in PS Formation
Any models attempting to describe the overall formation mechanism of PS must con-
sider the fundamental electrochemical reactions in three essential aspects: (1) nature of
reactions, reactants, products, intermediates, number of steps, and their sequences, (2)
nature and rate of charge transport in the different physical phases at the silicon/elec-
trolyte interface, and (3) spatial and temporal distributions of reactions and the cause
of such distributions. Also, the models have to take into account every factors deter-
mining the PS morphology such as doping type and concentration, orientation of
silicon, HF concentration, pH, illumination light wavelength and intensity, current
density and potential.
The first and second aspects have been extensively investigated and reasonably
well characterized as can be appreciated by the results described in Chapters 2-5. The
third aspect, on the other hand, is still not well characterized due to its complexity.
However, it is this aspect that determines the specific morphology of PS formed under
given conditions. It is not yet clear how this aspect can be described with an integral
model in accordance with the morphology of PS. Nevertheless, the spatial and tempo-
ral distributions of reactions are governed by the elements and events involved in the
system. The following is an attempt to qualitatively sort out the physical schemes of
these elements and events and their relative importance in the context of the overall
system.
Effect of Radius of Curvature.
The bottom of all pores is curved and the cur-
vature of a semiconductor surface affects the field at the surface. For an interface with
a spherical shape as illustrated in Fig. 8.64, the potential and field in the semiconduc-
tor can be calculated by solving Poisson's equation
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